Compensated phototube amplifier



June 4, 1957 G.' B. woRTHEN COMPENSATED PHOTOTUBE AMPLIFIER V2snets-sheet 1 Filed Feb. 2a. 1951 1N V EN TOR G B WORTH EN AT ORNEY June4, 1957 G. B. WORTHEN Filed Feb. 2 8, 1951 2 Sheets-Sheet 2 Z LlJ (I [ID U llJ A E F IG. 2 a

OUTPUT OF FIRST STAGE B f\ f\ C U GRID VOLTAGE 0 'l' It?) Ld 2% 2 o auvmvroza G. B. WORTHEN ATTORNEY AUnited States Patent O COMPENSATEDPHOTOTUBE ANIPLIFIER George B. Worthen, New York, N. Y., assignor to TheWestern Union Telegraph Company, New York, N. Y., a corporation of NewYork Application February 2.8, 1951, Serial No. 213,135

9 Claims. (Cl. 178-7.1)

The present invention relates to ampliiiers for use with phototubes andmore particularly to phototube amplifiers designed to providesubstantially constant average output signal levels.

The average output signal level of an amplifier circuit arranged toamplify the output of a phototube will tend to vary materially due tochanges in supply voltage and an alternating supply voltage. '1` he mostsigniiicant cause of this change in average or unmodulated level is thechange in illumination provided by the exciter lampi The change inillumination may be due to variations in the exciter lamp supplyvol-tage and to the presence of alternating components in the supplyvoltage. The term average level, as used in the specification and claimsrefers to the level when the carrier is not modulated with intelligence.

Changes in supply voltage, which may be caused, for instance, by changesin the load coupled to the supply line, generally resul-t in slowchanges in average illumination provided by the exciter lamp 'becausesuch load changes are usually infrequent in character. Where the supplyvoltage is alternating or has alternating components, the illuminationprovided by the exciter lamp varies at a rate double that of thealternations, but this rate is generally low with respect to themodulation frequencies iso that the resultan-t phototube outputvariations may generally be considered as changes in average level.

Changes in supply voltage also tend to alect amplifier average outputlevel by changing tube heater voltages and phctotube operatingpotentials.

ln many systems employing phototubes, the average output level must bemaintained substantially constant. For instance, in telegraphiccommunication rby facsimile, it is of great importance that theunmodulated carrier level be maintained very constant, preferably withina decibel. ln a typical facsimile transmitter employing a gas typephototube, a -line voltage change from 100 volts to 130 volts resultedin an unmodula-ted carrier level change of approximately 12 decibels. Ofthis 12 decibels, 8.5 decibels may be assigned to exciter lampillumination change, 2.5 decibels to the phototube, and 1.0 decibel tothe amplifier.

The effects of supply voltage variations can be minimized by using apower source having a relatively constant voltage, such ais a battery ora regulated power supply. Similarly, the elec-t of alternatingcomponents in the exciter lamp supply can be minimized by using abattery, a well iltered power supply or a high frequency power supply.Such expedients are, however, expensive and bulky and are not wellsuited for use in -a compact and inexpensive facsimile transmitter ortransceiver.

Accordingly, it is an object of the invention to provide a phototubeamplifier circuit having an average output level substantiallyindependent of supply voltage variations and supplyv voltagealternations.

More particularly, it is an object of the invention to provide a compactand inexpensive phototube amplifier for use in a -facsimile transmitteror transceiver.

Another object of the invention is to provide a facsimile phototubeamplifier in which the ampli-lied unmodulated carrier level issubstantially independent of supply voltage variations.

`Still another object of the invention is to provide a facsimilephototube amplifier -in which the -amplied unmodulated carrier level issubstantially free of amplitude variations .produced bypalternations ofthe supply voltage.

'Furth-er objectsof the invention will appear from the followingdescription.

According to the invention, Ithese objects are -achieved by providing aphototube amplifier circuit wherein there is supplied a iirst regulatedvoltage .for the phototube and an adjustable biasing potential for oneor more of the amplifier tubes, the biasing potential being constituted'by the difference in potential between a second regulated voltage and-a voltage varying with the supply voltage and wherein a modulation ofthe same frequency as the exciter lam'p illumination iluctuatons and inphase opposition thereto -is employed to cancel the effect of theillumination lluctuations onthe amplifier unmodulated or average outputlevel.

The invention will now be described in greater detail with reference tothe appended drawing in which:

IFig. 1 illustrates a facsimile transmitter circuit arrangement inaccordance with the invention for providing a substantially constantunmodulated carrier level substantially independent of supply voltagevariations in which the etle'cts of alternating current in the supplyvoltage areisubstantiarlly eliminated; and

IFig. 2 shows an' amplifier tube transfer characteristic curve forexplaining the operation of Ithe circuit ot Fig. 1.

Referring now to the drawing, and more particularly to Fig. 1, there isshown a phototube I10 having a cathode 11 connected -to ground and ananode 12 coupled to a control grid 13 of an electron discharge tube 14through a capacitor 15. v A regulated direct operating potential forphototu'be 10 is derived from a series combination of resistanceelements 16, 17 and 18 by coupling anode 1-2 to the junction ofresistors 16 and 17 through a resistor 19. The direct potential acrossthe series combination' of resistors 16, 17 and 18, as well as thepotential supplied to phototube 10, is maintained substantially constantby shunting the series combination of resistors 16, 17 and '18 with apair of series connected neon tubes 20 and 21. Other tubes havingvoltage regulator characteristics could, of course, be used in place ofneon tubes. The free terminal of tube 20 is connected to the free end ofresistor 16, while the free terminal of tube 21 is connected to the freeend of resistor 18 and to ground.

A high positive direct voltage is .applied to the free end of resistor16 through a resistor 22 which is, in turn, coupled to the tapping of apotentiometer 23. Potentiometer `23 forms part of a series loadresistance circuit including potentiometer 23, a resistor 24, and apotentiometer 25. The free ends of potentiometers 23 and 25 -areconnected, respectively, to the high and low potential terminals of asmoothing tlter 26. Smoothing iilter 26 is, in turn, coupled to thecathodes of a full wave rectiiier tube 27 through a secondary winding 28of a power transformer 29. The anodes of rtube 27 are coupled to a highvoltage secondary winding 30 of transformer 29. Transformer 29 is alsoprovided with two additional secondary windings 31 and 32 forenergizing, respectively, an exciter lamp 'EL and the tube heaterterminals. Exciter lamp EL is arranged to provide illumination forphototube 10. The light from exciter lamp EL is directed onto a copysheet C having markings M,

and is retiected onto cathode 1,1. A chopper disc D is employed toprovide relatively high frequency interruptions of the light impingingon phototube 10. The various secondary windings of transformer 29 areenergized from a primary wind-ing 33 coupled to power line PL. Powerline PL might be, for instance, a conventional 60 cycle 110-120 voltpower main. Power line PL could, of course, be constituted by othersources of alternating power. Furthermore, the invention is equallyIapplicable to sources of direct voltage. Where a direct power voltageis employed, it may be applied directly, or through a voltage divider,to resistors 23, 24 and 25 and to exciter lamp EL.

Cathode 34 of tube 14 is coupled to a tapping of potentiometer 25,thereby applying a relatively low positive potential to the cathode. Thevalue of this positive potential, which constitutes one of the biasvoltages for tube 14, will vary with variations in the supply voltageapplied to transformer 29. The other bias voltage for tube 14, also apositive voltage, is applied to grid 13 through a resistor 35 connectedto the junction of resistors 17 and 18. Due to the regulator action oftubes 20 and 21, this latter bias voltage will remain substantiallyconstant.

A suppressor grid 36 of tube 14 is connected to cathode 34, while ascreen grid 37 is coupled to cathode 34 through a decoupling capacitor38. An anode 39 and screen grid 37 are supplied with positive operatingpotentials from the junction of potentiometer 23 and resistor `24through resistors 41 and 42, respectively. Ihe junction of resistor 24and potentiometer 23 is coupled to ground through a by-pass capacitor40.

The amplified output of tube 14 is applied to control grid 43 of asecond amplilier tube 44 through a capacitor 45. Control grid 43 iscoupled to ground through a resistor 46. Cathode 47 of tube 44 iscoupled to ground through a biasing resistor 48 and a by-pass capacitor49. Anode 50 of tube 44 is supplied with a positive operating potentialthrough an anode resistor 51.

The output of tube 44 is applied to control grid 52 of a third amplifiertube 53 through a coupling capacitor 54. Control grid 52 is connected toground through a resistor 55. Cathode 56 of tube 53 is coupled to groundthrough a biasing resistor 57 and a by-pass capacitor 58. Positiveoperating potential for anode 59 of tube 53 is applied thereto through aprimary winding 60 of an output transformer 61. The amplifier outputsignal is derived from terminals OT coupled to a. secondary winding 62of transformer 61.

The grid bias of tube 14, which is equal to the di'erence between theregulated positive voltage applied to grid 13 and the unregulatedvoltage applied to cathode 34, is selected to provide the desired tubeoperating point at normal power line voltage. If the power line voltageincreases, the voltage applied to cathode 34 will rise accordingly,thereby increasing the effective negative grid bias of tube 14.

Such an increase in power line voltage will be accompanied by anincrease in the illumination supplied by exciter lamp EL, so that theaverage output level of phototube will increase. If the increase 'innegative bias applied to tube 14 reduces the gain thereof by the properamount, the decrease in gain will compensate for the increase in averagephototube output, so that the average signal level at terminals OT willremain substantially unchanged. A decrease in power line voltage willresult in a decrease in exciter lamp illumination and phototube averageoutput and also in a decrease in negative bias of tube 14 and increasein gain thereof, thereby compensating for the decrease in averagephototube output.

In order properly to compensate for line voltage variations over a givenrange, it is necessary properly to choose the normal operating point oftube 14. First, it is necessary to choose an operating point at whichtube gain will vary appreciably with bias. For a remote cut-ott tube,such as, for instance, th'e type 6SK7, the bias range over which suchgain control is possible is relatively large. With a sharp cut-off tube,such as, for instance, the type 6517, the bias range over which suchgain control is possible is limited to the relatively small bias rangein which the tube characteristic has substantial curvature.

Referring to Fig. 2, which is a plot of plate current versus gridvoltage, the curve shown may be considered as the non-linear portion ofa transfer characteristic of either a sharp or remote cut-oli? tube.Assuming, for purposes of illustration only, that the power line voltageis normally ll5 volts and that it may vary between volts and 130 volts,points A, B and C correspond, respectively, to the effective biaspotentials applied to tube 14 for the line voltages 100, 115 and 130volts. The signal voltage applied to grid 13 will have relatively low,medium andhigh values corresponding, respectively, to line voltages of100, and 130 volts because ot the effect of line voltage on exciter lampbrilliance.

If point B, which is the operating point for normal line voltage, isproperly chosen, the output voltages corresponding to points A, B and C,respectively, will each have substantially the same values because ofthe curvature of the tube characteristic. Intermediate pointscorresponding to intermediate line voltages, which may not be as closelycompensated, will nevertheless yield substantially constant outputvoltages.

When using a sharp cut-off tube, only small signal voltages may beamplified without distortion when the tube is operated in thisnon-linear range. However, in facsimile circuits, as wall as in mostphototube applications, the signal voltage to be amplied is relativelysmall.

It is important that grid 13 be returned to a regulated positive voltageand not to ground potential. The reason for this can be seen from thefollowing example, given by way of illustration. Assume that a change inbias of 2.5 volts is required to change the tube gain by 4 decibels andthat a line voltage change of :1 -10% required such a change in gain forcompensation. Assume also that changes in unregulated (cathode) voltageare proportional to changes in line voltage. A ten percent change in theunregulated voltage would have to correspond to a change in grid bias of2.5 volts. The normal unregulated voltage VI therefore would be 0.1VI=2.5 Vlr- 25 A 25 volt positive potential on the cathode would resultin the tubes being cut-ofi at all times. However, if a fixed positivepotential of proper value is applied to the grid, the tube will not bccut otl. Even if tube 14 were not cut off by the bias developed, thegain thereof would have an inordinatcly low value. ln the exampleassumed, a regulated potential on the grid of 2l volts would cause thenet bias, for a cathode voltage of 25 Volts, to be -4 volts, a morepractical value. The actual regulation characteristic for a given tubewill depend upon the voltages and circuit parameters selected.

The illumination provided by exciter lamp EL will vary at a ratedetermined by the frequency of the voltage supplied from power line PL.Assuming a 60 cycle supply, the current supplied to exciter lamp EL willtraverse a positive and a negative set of values during each cycle.Since the exciter lamp presents an impedance substantially resistive incharacter and since it will light on both positive and negative halfcycles, the illumination will vary at a rate equal to twice the supplyfrequency, or cycles per second. Since the filament of the exciter lamphas considerable thermal inertia, the maximum and minimum lightintensities will not occur simultaneously with the maximum and minimumvoltage values, respectively. Thus there is a time or phase lag betweenvoltge and light alternations. The actual amplitude of the lightintensity variations will not be very :afmeren-o ,to preventi 4completeextinguislrment ofi thellamp.

Howeven the light-variationsfactually-produced will .harmonically indensity on the recording copy sheet. vThese bars, which actually appear..as variations in back* ground level, are produced because phototube ;10is=..i1nable to distinguish between;variationsinfexciter light-.and4variations indensity of thertransmitted copy. lThe-'light f potentialhaving: said wgiven gpolan'ty, means-whereby the second -biasingpotential: variescontinuously and uninter- .ruptedly inproportiontotvariations in magnitude of'said voltage supply, means: toapp ly.said second biasing poten- Atial to said cathodeelectrode therebyto vary the vgain of said discharge tube in -a sense opposite-Ato themagnitude Yvariations.l of saidsupply-voltage, andmeans coupled to A.theanode of said discharge. tube to jderive therefrom .a fmodulated signalvoltage having alvalue proportional to variations may be consideredasproducingmodulation of p10 Said 'modlliatingvariaions and Substantiallyindependent thefacsimile carrier at doublc; p0wer,line'frequency If .achopper producing a 5000 cycle carrier,is;e`mpl0yed, the 5000 cyclecarrier would be modulated..with.a 120 cycle signal for a powery linefrequency 0f60 cycles.

Inaccordance with the invention, thismodulation component is suppressedby combining it .with a modulation component of equal amplitude 'but inphase opposition thereto. For this purpose, a low' pass filter 70 is,coupled to the center tap of secondary Wiudiugs to `provide a .voltageatdouble power linerfrequency. Assuming a 60 cycle supply, low passtilter 70 should pass'l'20cycles andsuppress all other harmonicsintheutput of rectifier 27. The-full wave rectiticationprovidedby tubeZ7 will result in a predominant ripple-frequency of 120 cycles..Filter'7-0could also be realizedas i-a bandpass' filter 'designed toaccept 120 cycles. 'I`l 1e outp ut'of\filter.7 0 is applied .to a phaseshifting network 71 comprising a potentiometer 72 and capacitors 73, 74-and J'15. The phase of the voltage applied to network 71 may be variedof variations in supply voltage to said exciter lamp.

2. A transducer circuit arrangement for converting modulating variationsin intensity of illumination from ,en -.exciter lamp Iinto a vmodulatedsignal voltage having an unmodulated carrier amplitude-substantiallyindependent of variations in supplyvvoltage to said exciter lamp,comprising an exciter lamp -having a-variable voltage supply forproducing avariable source of illumination, a phototube disposed in thepath of said illumination and -having an output voltage responsive tovariations thereof, an electron discharge tube having cathode, controlgrid and lanode electrodes. and having-atransfer characteristic .with anon-linear portion, means to .apply said output .voltage to said controlAgrid-electrode, a source of a first -substantially constantpositivebiasing-potential, means .to apply saidifirst biasing potential to saidcontrol grid lelectrode, a source of a second positive biasingpotential, ,meansWhereby thesecond biasing` potential variescontinuously andY uninterruptedlyl-in; proportion to variations throughsubstantially 180 by Aadjusting vthe=tapping of :30 illmagniilde 0fsaid'voltagsuppiy,Said SeCDnd biasing potentiometer 72. The tappingof'potentiometer'72'is coupled to a potentiometer 76, the tapping ofwhich is coupled through a capacitor 77 to v,a'control grid 78. of amodulator tube '79. 'Control grid 4.78 isfalso coupled 'to groundthrough a resistor 80. `Cathode-81isA coupled to tground through abiasing resistor-82 and-aby-passcapacitor 83. Anode `84 is coupled toanode 12 of phototube 10 through a resistor. 85.

.The phase and amplitude of the voltage from filter v70 are adjusted bymeans of thetappings on potentiometers 72 and 76, respectively, so as toproduce a ,modulation component equal in amplitude andin phaseoppositionto the modulation component produced b y.the illumination fluctuationsof lamp EL. The two modulation components therefore substantially cancelout. A band rejection iilter 86 is interposed between phototube 10.andtube 14 to suppress any .unmodulated 120 cycleripple present because ofthe use of a single ended modulator.

It is seen that by using this circuit, -theoutput signal developed atterminals OT will have asubstantially constant average value without apower supply component. In a facsimile system, this corresponds to asubstantially constant background or spacing level.

While the invention has been described in aparticular use thereof and inparticular embodiments, it isv not dei sired that it be limited thereto,for obvious modifications thereof will occur to those skilledvin the artwithout departing from the spirit and scope-of the invention as setforth in the appended claims.

What is claimed is:

1. A transducer circuit arrangement for converting modulating variationsin intensity of illumination from an exciter lamp into a modulatedsignal voltage having an unmodulated carrier amplitude substantiallyindependent of variations in supply voltage to saidexciter lamp,comprising an exciter .lamp having .a variable voltage supply forproducing a variable sourceof illumination, a phototube disposed in-thepath of said illumination and having an output voltageresponsive tovariations thereof, an electron discharge tube having cathode andcontrol grid electrodes and having an anode,.means to apply said outputvoltage 'to said control grid electrode, a source of a firstsubstantially constant biasing :potentialhaving a given polarity,Imeansto apply said first biasing potential to said control gridelectrode, a 1source of asecond biasing potential .havingia-rvaluegreater than said tirst biasing potential ltherebyr toproduce operationof said tube about --a,.point 4within said-non-linearf-portion ofasaid.transfer :characteristic,means .tofapply said secondpositive biasingpotential to-said cathode electrode thereby to vary the gain of said'dischargetuberinafsense opposite tothe `magnitude variations-ofsaid-'supply voltage, and means ;coupled'to the anode of said dischargetube to derive u therefrom a modulated signal voltage having a valueproportionalto said modulating variations and Asubstan- .tiallyindependent of variations in supply voltage to said exciter lamp.

3. A transducer' circuit arrangement for converting 'modulatingvariations in intensity of illumination from :anrexciter lamp-into amodulated signal voltage having an unmodulated carrier amplitudesubstantially independ- ,entof variations in supply voltage tosaidexciter lamp, comprising :an exciter lamp having a variable voltagevsupply.forproducing -a variable source of illumination, Va phototubedisposed in the path of said illumination and having an output voltageresponsive to variations thereof, -anelectron discharge tube havingcathode, control grid and anode electrodesfmeans to apply said outputvoltage to said control grid electrode, a source of a firstsubstantially constant positive potential, means to apply a firstportion of said first potential to said phototube as an operatingpotential therefor, means vto apply-a second portion of said firstpotential to said control grid electrode as a first biasingpotentialtherefor, a source of a second positive biasing potential,means .whereby the second biasing potential varies continuouslyanduninterruptedly inproportonto variationsin-magnitude of said voltage-supply,means to apply said second positive biasing poten tial to saidcathode-electrode thereby .to vary the gain of .said dischargetube in asense opposite to the magnitude variations of said supply voltage, andmeans coupled to .the anode of said dischargetubezto lderive'therefrom a.modulated signal voltagehaving a value proportional to said modulatingvariations and substantially independent of variations in supply voltageto said exciter lamp.

4. In a facsimile `transmitter comprising an 4exciter llamp having a-voltage supply forproducing a variable source of illumination, a copysheet having markings Ythereon and arranged-to direct the light fromsaidvlamp .in .a given path and va phototubeffor converting varia-'tions in intensity of said light into a modulated facsimile signal andarranged in said path, a compensated amplifier comprising an electrondischarge tube having cathode, control grid and anode electrodes, meansto apply said facsimile signal to said control grid, a source of a firstsubstantially constant biasing potential having a given polarity, meansto apply said first biasing potential to said control grid, a source ofa second biasing potential having said given polarity, means whereby thesecond biasing potential varies continuously and uninterruptedly inproportion to variations in magnitude of said voltage supply, means toapply said second biasing potential to said cathode thereby to vary thegain of said discharge tube in a sense opposite to variations inmagnitude of the supply voltage to said exciter lamp, and means toderive from said anode an amplified facsimile signal having a backgroundlevel substantially independent of variations in magnitude of the supplyvoltage to said exciter lamp.

5. In a facsimile transmitter comprising an exciter lamp having avoltage supply for producing a variable source of illumination, a copysheet having markings thereon and'arranged to direct the light from saidlamp into a given path and a phototube for converting variations inintensity of said light into a modulated facsimile signal and arrangedin said path, a compensated multistage cascade amplifier, the firststage of said amplifier comprising an electron discharge tube havingcathode, control grid and anode electrodes, means to apply saidfacsimile signal to said control grid, a source of a first substantiallyconstant positive potential, means to apply a tirst portion of saidfirst positive potential to said phototube as an operating potentialtherefor, means to apply a second portion of said first potential tosaid control grid as a first biasing potential therefor, a source of asecond positive biasing potential, means whereby the second biasingpotential varies continuously and uninterruptedly in proportion tovariations in magnitude of said voltage supply, means to apply saidsecond biasing potential to said cathode thereby to vary the gain ofsaid discharge tube in a sense opposite to variations in magnitude ofthe supply voltage to said exciter lamp, and means to derive from saidamplifier an amplified facsimile signal having a background levelsubstantially independent of variations in magnitude of the supplyvoltage to said exciter lamp.

6. A transducer circuit arrangement, comprising an exciter lamp, avariable source of supply voltage for said exciter lamp, said supplyvoltage having an alternating component producing a first modulationcomponent in the light from said exciter lamp, a phototube disposed inthe path of light from said lamp, means interposed in the path betweensaid lamp and said phototube to vary the intensity of illuminationimpinging on said phototube to thereby produce a second modulationcomponent in the light from said lamp impinging on said phototube, meansto derive from said phototube a modulated signal voltage having firstand second signal modulation components proportional, respectively, tosaid first and second modulation components, an amplifier electrondischarge tube having cathode, control grid and anode electrodes, meansto apply said signal voltage to said control grid, a source of a firstsubstantially constant biasing potential having a given polarity, meansto apply said first biasing potential to said control grid, a source ofa second biasing potential having said given polarity and having amagnitude proportional to the magnitude of the supply voltage for saidexciter lamp, means to apply said second biasing potential to saidcathode thereby to vary the gain of said amplifier tube in a senseopposite to magnitude variations of the supply voltage for said exciterlamp, means to derive from said supply voltage a compensating voltageequal in magnitude and in phase opposition to said first signalmodulation component, means to modulate said compensating voltage onsaid signal voltage thereby to suppress said first signal modulationcomponent, and means coupled to the 'anode of said discharge tube t0derive therefrom an amplified modulated signal voltage having a valueproportional to said'second modulation component and substantiallyindependent of magnitude variations and alternating components of thesupply voltage for said exciter lamp. Y

7. A transducer circuit arrangement, comprising an exciter lamp, avariable source of supply voltage for said exciter lamp, said supplyvoltage having an alternating component producing a first modulationcomponent in the light from said exciter lamp, a phototube disposed inthe path of light from said lamp, means interposed in the path betweensaid lamp and said phototube to vary the intensity of illuminationimpinging on said phototube to thereby produce a second modulationcomponent in the light from said lamp impinging on said phototube, meansto derive from said phototube a modulated signal voltage having firstand second signal modulation components proportional, respectively, tosaid first and second modulation components, an amplifier electrondischarge tube having cathode, control grid and anode electrodes, meansto apply said signal voltage to said control grid, a source of a firstsubstantially constant positive biasing potential, means to apply saidfirst biasing potential to said control grid, a source of a secondpositive biasing potential having a magnitude proportional to themagnitude ofthe supply voltage for said exciter lamp, means to applysaid second biasing potential to said cathode thereby to vary the gainof said amplifier tube in a sense opposite to magnitude variations ofthe supply voltage for said exciter lamp, means to derive from saidsupply voltage a compensating voltage equal in magnitude and in phaseopposition to said first signal modulation component, means to modulatesaid compensating voltage on said signal voltage thereby to suppresssaid first signal modulation component, and means coupled to the anodeof said discharge tube to derive therefrom an amplified modulated signalvoltage having a value proportional to said second modulation componentand substantially independent of magnitude variations and alternatingcomponents of the supply voltage for said exciter lamp.

'8. A transducer circuit arrangement, comprising an exciter lamp, avariable source of alternating supply voltage for said exciter lamp, thealternations of said supply voltage producing a first modulationcomponent in the light from said exciter lamp, a phototube disposed inthe path of light from said lamp, means interposed in the path betweensaid lamp and said phototube to vary the intensity of illuminationimpinging on said phototube to thereby produce a second modulationcomponent in the iight from said lamp impingng on said phototube, meansto derive from said phototube a modulated signal voltage having firstand second signal modulation components proportional, respectively, tosaid first and second modulation components, an amplifier electrondischarge tube having cathode, control grid and anode electrodes, meansto apply said signal voltage to said control grid, a source of a firstsubstantially constant positive biasing potential, means to apply saidfirst biasing potential to said control grid, a source of a secondpositive biasing potential having a magnitude proportional to themagnitude of the supply voltage for said exciter lamp, means to applysaid second biasing potential to said cathode thereby to vary the gainof said amplifier tube in a sense opposite to magnitude variations ofthe supply voltage for said exciter lamp, means including a phaseshifting network to derive from said supply voltage a compensatingvoltage equal in magnitude and in phase opposition to said first signalmodulation component, means to modulate said compensating voltage onsaid signal voltage thereby to suppress said first signal modulationcomponent, and means coupled to the anode of said discharge tube toderive therefrom an amplified modulated signal voltage having a valueproportional to said second modulation component and substantiallyindependent of magnitude variations and alternations of the supplyvoltage for said exciter lamp.

9. A transducer circuit arrangement, comprising an exciter lamp, avariable source of supply voltage for said exciter lamp, said supplyvoltage having an alternating component producing a rst modulationcomponent in the light from said exciter lamp, a phototube disposed inthe path of light from said lamp, means comprising a facsimile copysheet interposed in the path between said lamp and said phototube tovary the intensity of illumination impinging on said phototube tothereby produce a second modulation component in the light from saidlamp impinging on said phototube, means to derive from said phototube amodulated signal voltage having rst and second signal modulationcomponents proportional, respectively, to said first and secondmodulation components, an amplifier electron discharge tube havingcathode, control grid and anode electrodes, means to apply said signalvoltage to said control grid, means to derive from said supply voltage arst substantially constant positive biasing potential, means to applysaid rst biasing potential to said control grid, means to derive fromsaid supply voltage a second positive biasing potential having amagnitude proportional to the magnitude of l0 the supply voltage forsaid exciter lamp7 means to apply said second biasing potential to saidcathode thereby to vary the gain of said amplifier tube in a senseopposite to magnitude variations of the supply voltage for said exciterlamp, means to derive from said supply voltage a compensating voltageequal in magnitude and in phase opposition to said rst signal modulationcomponent, means to modulate said compensating voltage on said signalvoltage thereby to suppress said rst signal modulation component, andmeans coupled to the anode of said discharge tube to derive therefrom anamplified modulated signal voltage having a value proportional t0 saidsecond modulation component and substantially independent of magnitudevariations and alternating components of the supply voltage for saidexciter lamp.

References Cited inthe tile of this patent UNITED STATES PATENTS2,228,560 Cox Jan. 14, 1941 2,236,172 Gray Mar. 25, 1941 2,336,673Cooley Dec. 14, 1943 2,420,058 Sweet May 6, 1947

